Electrolytic grinding chloride using an electrolyte including 2-imidazolidimethione



United States Patent 3,389,067 ELECTROLYTIC GRINDING CHLORIDE USING AN ELECTROLYTE INCLUDING Z-IMIDAZOLI- DIMETI-IIONE Mitchell A. La Boda, East Detroit, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Oct. 23, 1965, Ser. No. 504,073 7 Claims. (Cl. 204-443) This invention relates to electrolytic grinding processes and, more particularly, to electrolytes for use therewith.

In recent years electrolytic machining procedures for generating shapes, cavities and contoured surfaces have been developed and are more generally classified into one of two basic categories, the first being electrochemical machining and the second electrolytic grinding, a specialized application of the first. Electrolytic grinding is essentially an electrochemical deplating process which can be used on virtually any electrically conductive material. It is generally adapted to metal removal operations comparable to those performed by cutoff wheels, saws, and grinding or milling machines and the like, and uses equipment similar to conventional grinders except for the electrical accessories. About 95% of the metal removal results from electrolytic rather than mechanical action.

A particular version of an electrolytic grinding process is characterized by a fiow of electrolyte between the workpiece and a rotating grinding cathode wheel. The rotating cathode wheel comprises a conductive metal matrix having a plurality of nonconducting abrasive particles imbed-ded therein to provide non-conductive spacing between the workpiece and the cathodic matrix. Electric current is passed through the electrolyte to dissolve the anodic surfaces of the workpiece and the imbedded particles of the wheel abrade the surface to remove any irregularities resulting from nonuniform erosion or reaction product build up.

While aqueous solutions of individual inorganic salts, such as nitrates, cyanides, carbonates, hydroxides and nitrates have been used as electrolytes in electrochemical machining and grinding processes, none has offered any significant advantage over the now well accepted aqueous sodium chloride solution most commonly used today. However, regardless of what salt is chosen, an apparent problem with electrochemical machining and grinding processes using that salt singularly or in combination is overcut or wild-cutting which is the uncontrolled anodic dissolution of the workpiece in unwanted areas resulting in undesirable tapering of holes, rounding of edges, and the like. Such anodic dissolution can occur even in areas which are fairly well removed from the cathode. This wild-cutting or cutting in low current density areas which are bathed in the electrolyte but substantially rei moved from the cathode, has been substantially reduced in the prior art by the use of costly and time-consuming masking operations which isolate the areas to be machined by protecting the surrounding areas from the erosive effect of the electrolyte. These masking operations are frequently quite involved and require a high degree of skill to insure a satisfactory product. Likewise, additional steps subsequent to the machining steps are required to strip the workpiece of the mask. Additionally, the prior art has attempted to reduce wildcutting by designing special purpose electrodes and machines to meet individual and specialized machining requirements.

By my invention I have at least reduced, and in most cases actually eliminated, the need for recourse to the prior arts attempted resolutions.

It is, therefore, an object of my invention to provide a self-masking electrolyte for ECM.

It is a further object of my invention to provide an 3,389,067 Patented June 18, 1968 additive for existing ECM electrolytes which selectively inhibits anodic dissolution in unwanted areas.

It is a further object of my invention to effect a sharply contoured machining by a process utilizing a basic aqueous electrolyte containing known salts and improving same by adding thereto a compound which upon reaction with the work-piece forms a film thereover, which film inhibits or stops off electrolytic action or anodic dissolution in unwanted areas.

It is a further object of my invention to effect a sharply contoured electrochemical machining by a process utilizing aqueous electrolytes containing known salts and an additive consisting of electrochemical erosion inhibiting film forming -thiones such as Z-imidazolidinethione.

Further objects and advantages of the present invention will become apparent from the following detailed description of the invention.

My invention briefly stated involves adding certain electrochemical erosion inhibiting film forming -thiones to basic known ECM electrolytes. When added to these electrolytes the additives of my invention, and especially 2- imidazolidinethione, create an inhibited solution which effectively forms a heavy adherent film over the surface of the workpiece. The film retards or substantially eliminates electrochemical erosion in those areas protected by the Him. In a particular application of my invention a chloride electrolytic grinding electrolyte is modified by adding thereto Z-imidaZoIidinethione. The film formed is subsequently abraded away in those areas where electrochemical machining is to continue, hence presenting a limited uninhibited surface to unrestricted electrochemical action.

A significant advantage of 2-imidazolidinethione-not found with compounds of the same class or compounds which perform similarly as to the other aspects of the invention (forming an inhibiting film)-is the fact that the cathode remains almost perfectly clean, being completely free of smut or other contaminants which are normal incidents of such processes. This aspect alone makes the use of Z-imidazol-idinethione a significant improvement over electrolytes known heretofore, especially in those applications where heavy production electro-chemical machining is required, for it eliminates the requirement for extra cleaning or dressing of the cathode which necessarily follows the deposition of smut or other contaminants from the solution.

iy experience has been that using the additives of my invention, I can successfully machine metal samples ranging from the softer lower carbon steels (SAE 1008) to the harder low alloy steels (SAE 5160H). The additive of my invention is likewise applicable and effective for ferrous alloys wherein the iron content is 50% or more.

While the prefer-red electrolyte comprises a solution of 1.7%, by weight, of Z-imidazolidinethione, 19.7%, by weight of sodium chloride and the balance water, I have found that effective electrolytes can be compounded using from as low as 0.828%, by weight, Z-imidazolidinethione to as high as 4.74%, by weight, of 2-imidazolidinethione, the balance being a selected concentration of an aqueous sodium chloride solution wherein the concentration of the sodium chloride may vary from dilute to saturated. In this connection the lighter alkali metal (lithium, sodium and potassium) chlorides are preferred because they produce relatively neutral pHs, do not plate out or have a deleterious affect upon the cathode, and represent a source of inexpensive material. Likewise, while electrolytes dilute as to chloride ion are operative, as a practical matter there are no significant advantages to operating at the lower concentrations and, in fact, it is less desirable to do so when considering such factors as solution conductivity and the like. I have been successful in operating these electrolytes at voltages up to 40 volts and anode current densities from 5 to 500 amperes per square inch. However, it is to be expected that in some circumstances even higher current densities can be used.

Z-imidazolidinethione, while sufiiciently soluble in water at lower temperatures is not so readily soluble in sodium chloride solutions. Hence, I have found it desirable to prepare the subject electrolytes at approximately 150 F. and maintain them at this temperature during the machining. These slightly higher temperatures require the use of somewhat more elaborate equipment. However, the increased temperature may permit a higher chloride concentration which, in turn, may permit an increased cutting rate. This combined with the aforementioned cathode cleanliness benefits may offset the inconvenience and expense of higher temperature maintenance and equipment.

Generally speaking, tests were conducted utilizing a system wherein steel tube samples were brought up to a rotating sintered bronze diamond impregnated wheel. A gravity feed system kept the samples at the face of the wheel at all times. The feed system was such that an adjustable weight provided the capability of varying the pressures at which the samples would engage the wheel. It was found that to properly evaluate the inhibitive effects of my additives a minimum workpiece-to-wheel pressure should be employed in order to reduce the mechanical cutting component of the abrasive wheel. The electrolyte was pumped at a pressure of 9 p.s.i. through a bore in the workpiece and into the gap between the cathode and the workpiece at a rate of 0.25 gallon per minute. This gap was held constant at about .0025 inch by the spacer effect of the nonconductive diamond chips. Tube length decrease per unit time was used to determine metal removal rates. The tube ends were compared with those produced by electrochemically grinding similar samples under the same conditions with additivefree electrolytes.

The following are some specific examples encompassed within the scope of my invention:

EXAMPLE 1 An electrolyte comprising 3.37%, by weight, of 2-imidazolidinethione, 19.4%, by Weight, of sodium chloride and the balance water was used to machine a sample of an SAE 5160H alloy. The temperature of the electrolyte was maintained at 150 F. and a potential of 9 volts was applied. A current density of 500 amperes per square inch was maintained, resulting in a metal removal rate of 0.046 inch per minute. The sample exhibited a strong inhibiting film and displayed a finely machined area with relatively negligible overcut.

EXAMPLE 2 An electrolyte comprising 1.7%, by weight, of 2-imidazolidinethione, 19.7%, by weight, of sodium chloride and the balance water was used to machine a sample of an SAE 5160H alloy. The temperature of the electrolyte was maintained at 140 F. and the potential at 4.1 volts. A current density of 180 amperes per square inch was maintained, resulting in a metal removal rate of 0.040 inch per minute. The sample exhibited a strong adherent inhibiting film and displayed an excellently machined region.

EXAMPLE 3 An electrolyte comprising 2.8%, by weight, of 2-imidazolidinethione, 19.4%, by weight, sodium chloride and the balance water was used to machine a sample of an SAE 5160H alloy. The temperature of the electrolyte was maintained at F. and the potential at 4.1 volts. A current density of 200 amperes per square inch was maintained, resulting in a metal removal rate of 0.025 inch per minute. An adherent inhibiting film was formed thereby contributing to the production of a machining exhibiting relatively negligible overcut.

Other than abrasive means for the local removal of the inhibiting film of my invention may be employed, such as washing away with localized increased electrolyte flow, and/or a variety of sophisticated variations of these and others. Therefore, though my invention has been described in terms of a certain preferred embodiment, it is to be understood that others may be adapted and that the scope of my invention is not limited except by the appended claims.

I claim:

1. An aqueous electrochemical machining electrolyte consisting essentially of an alkali metal chloride and 2-imidazolidinethione, wherein the concentration of said Z-imidazolidinethione is from about 0.828% to about 4.74% by weight, with the balance being said alkali metal chloride in aqueous solution.

2. An electrolyte in accordance with claim 1 wherein said alkali metal is from the group consisting of lithium, sodium and potassium.

3. An electrolyte in accordance with claim 1 wherein the concentration of said 2-imidazolidinethione is about 1.7%, by weight, the balance being an aqueous solution of alkali metal chlorides.

4. An aqueous electrochemical machining electrolyte consisting essentially of about 1.7%, by weight, of Z-imidazolidinethione, about 19.7%, by weight, of sodium chloride and the balance water.

5. A process for electrochemically machining ferrous metals and alloys thereof comprising the steps of establishing said metal as the anode in an electrochemical cell, orienting an electrolytic grinding cathode adjacent to said metal, bathing the junction between said cathode and said steel in an aqueous electrolyte consisting essentially of an alkali metal chloride and 2-imidazolidinethione, passing an electric current through said steel, electrolyte and cathode, whereby an electrochemical erosion inhibiting film is formed, and selectively removing said film, whereby electrochemical machining can thus selectively continue.

6. The process as defined in claim 5 wherein the concentration of said Z-imidazolidinethione is from about 0.828%, by weight, to about 4.74%, by weight.

7. The process as defined in claim 5 wherein the concentration of said alkali metal chloride is about 19.7%, by weight, and the concentration of said Z-imidazolidinethione is about 1.7%, by weight, the balance being water.

References Cited UNITED STATES PATENTS 3,058,895 10/1962 Williams 204-443 3,130,138 4/1964 Faust et al. 204-143 3,284,327 11/1966 Maeda et al. 204-143 2,939,825 6/1960 Faust et al. 204-142 ROBERT K. MIHALEK, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,389,067 June 18, 1968 Mitchell A. La Boda It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, lines 41 and 43, "steel", each occurrence, should read metal Signed and sealed this 23rd day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, J r.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

5. A PROCESS FOR ELECTROCHEMICALLY MACHINE FERROUS METALS AND ALLOYS THEREOF COMPRISING THE STEPS OF ESTABLISHING SAID METAL AS THE ANODE IN AN ELECTROCHEMICAL CELL, ORIENTING AN ELECTROLYTIC GRINDING CATHODE ADJACENT TO SAID METAL, BATHING THE JUNCTION BETWEEN SAID CATHODE AND SAID STEEL IN AN AQUEOUS ELECTROLYTE CONSISTING ESSENTIALLY OF AN ALKALI METAL CHLORIDE AND 2-IMIDAZOLIDINETHIONE, PASSING AN ELECTRIC CURRENT THROUGH SAID STEEL, ELECTROLYTE AND CHATHODE, WHEREBY AN ELECTROCHEMICAL EROSION INHIBITING FILM IS FORMED, AND SELECTIVELY REMOVING SAID FILM, WHEREBY ELECTROCHEMICAL MACHINING CAN THUS SELECTIVELY CONTINUE. 